Internal aquarium water filters.
Internal filters (also known as submersible filters) have the decisive advantage of being located inside the aquarium, meaning that no water pipes or hoses need to be run outside of it. This affects their size, ease of maintenance, and the role they can play in the aquarium. Most internal filters are a small pump that circulates aquarium water through a special container filled with some filter material.
As the filter collects dirt from the aquarium, its performance drops, and it becomes harder for the pump to push water through. As it clogs, the filter material must be washed. When cleaning the filter, make sure that no dead leaves or sand have entered the pump’s intake ports. Debris must be removed as it prevents the device from working properly.
Due to this, internal filters are mostly used as mechanical filters, i.e., those that remove various kinds of debris from the water, including quite small particles (less than 1 mm). Internal filters driven by an air compressor have relatively low power. They cannot clear heavily clouded water. When necessary, special cartridges, such as carbon or zeolite ones (for advanced filtration), can be inserted into such filters.
Internal filters are generally suitable for small aquariums – up to approximately 180 liters – and are widely popular because they are versatile, widely available, and occupy the lower and middle price ranges on the market. They cannot be recommended as biological or chemical filters, although most aquariums use precisely these – internal mechanical rather than external biological filters. This is probably the result of their low cost on the one hand, and aquarium owners’ lack of knowledge combined with a certain tradition on the other. Disadvantages include a relatively small filter volume and an additional foreign object in the aquarium that needs to be properly camouflaged.
The main functions performed by an internal filter in an aquarium are: • mechanical filtration; • biological filtration; • chemical filtration; • creation of 
water circulation; • aeration. The filter does not always have to perform all of the above functions at once. Which function is primary and which is secondary depends on the specific situation in each individual aquarium and the tasks the aquarist is solving. In a cichlid aquarium, internal filters typically act as an additional mechanical filter and are sometimes used for aeration. Along with this, they are also used in various types of small temporary aquariums (breeding tanks, quarantine tanks).
Unfortunately, the common traits of all internal filters are limited pump performance (up to 1200 l/h) and a small volume of filter material (up to 700 sq. cm). In this regard, regardless of what is written in the manufacturer’s manual (“150-200 liters” or “over 200 liters”), a two-hundred-liter aquarium is generally the limit of an internal filter’s capabilities.
At the same time, internal filters compare favorably with power heads due to their more aesthetic appearance, ease of maintenance, and, importantly for an indoor aquarium, relatively low noise level; compared to external filters, they feature a low price and no need to allocate extra space outside the aquarium. Internal filter pumps can also be useful for pumping water. It is not necessary to bail it out with a bucket from one aquarium to another; this can be done using a hose fitted tightly onto the pump outlet.
Internal aquarium filters are the most frequently used, and generally, they all share the same design. In most internal filter models, the body consists of 2 halves. The lower half is removable, shaped like a hollow cylinder (or consists of two or more assembled sections), into which the filter material is placed. To allow water to wash over it evenly, slots are made in the cylinder, and a grid tube is inserted inside the device. This entire structure is attached with suction cups to the inside of the aquarium and is completely submerged in water.
Foam rubber should be recognized as the best filtering material for such a product: due to its tiny internal cavities, saprophytes can live and multiply on it. Only water-resistant foam rubber should be used for filters, as regular foam quickly turns to dust. The fine-pored and fibrous materials used in some models are not as efficient: after some time, they clog and can greatly weaken the water flow through the filter. Some models also have a small compartment for loose media or filter floss.
Activated carbon is usually used for chemical water purification. In fact, chemical purification is not a mandatory link in every aquarium. But occasionally fish do get sick, and medications have to be added to the water. After a course of treatment, it is absolutely necessary to pull the remnants of medicine out of the aquarium, and carbon is simply irreplaceable here. Therefore, it is better to purchase an internal filter with at least one additional section for possible carbon filling.
In the upper part of the device is a centrifugal pump that drives aquarium water through the filter material. The pump is designed as an electric motor, whose stationary winding – the stator – is hidden in a sealed housing, and the rotating rotor is integrated with the impeller that pumps the water. Depending on the size of the electric motor itself, as well as its power and rpm, pump performance can range from 100 l/h to 3,000 l/h. The filter creates the water current necessary for many inhabitants, mixing and also oxygenating the water. It includes several water flow direction settings.
Standard operating principle of an internal filter. The strong jet expelled by the pump is directed along the aquarium wall, establishing a circular water movement in the tank that brings dirt particles to the filter. The water stream expelled by the filter reaches the opposite wall, bounces off it, and brings suspended particles to the filter cylinder slots, where they are sucked in (i.e., the current must be strong enough, but not too strong to prevent secondary reflection, this time from the filter itself, blowing the dirt away). Then inside the cylinder through the filter material and, once cleaned, out through the pump output nozzle back into the aquarium.
Good internal pumps have a number of additional accessories, listed in order of increasing practicality and cost: • Tube for supplying 
atmospheric air. As a result, small air bubbles will come out of the opening along with the water. Thus, the pump helps oxygenate the water. Many beginners who buy an aquarium with a tightly closed lid make a mistake: they leave the outer end of the tube under the hood. As a result, the water stream is enriched with the very air that has already given up all its oxygen and managed to become saturated with carbon dioxide. In aquariums with a solid hood, the filter’s air intake tube must be led outside, ensuring a constant supply of fresh air; • A rotating outlet tube that allows directing the water jet in different planes; • An attachment for the outlet tube allowing the jet to be directed up or down. By angling the water flow down, you improve aeration, and by directing it up, you create surface agitation; • A flow power regulator that allows reducing the water flow or, conversely, making the pump work at full capacity; • A switch that allows reversing the flow direction; • A flow power indicator that visually shows how strong the jet or even the pump performance in l/h is; • Built-in heater.
However, despite the fact that the potential of internal mechanical filters is much lower than that of external ones, and they do spoil the aquarium design, they are still in demand and reasonably so. In small aquariums, it is entirely logical to use a small internal filter. In picture-frame aquariums, no other filters can be placed. When raising fry or keeping small fish, using an internal filter is often sufficient, because bacteria also settle on the filter sponge, purifying the water from metabolic products, meaning they also act as a small and weak, but nevertheless, biofilter.
In addition to the main functions – filtration and water circulation in the aquarium – internal filters can provide additional water aeration. Air drawn from the water surface is fed through a tube into the filter bell or directly onto the unit’s turbine, and then sprayed inside the aquarium.
The main characteristic of an internal filter is pump performance, measured in liters per hour. For effective biological filtration, it is enough to have 1.5-2 turnovers per hour. At the same time, one should remember another important characteristic that is sometimes “forgotten” not only by users but also by filter manufacturers (omitted from specifications), namely the volume of filter material. However, the quality of biofiltration also depends on the flow distribution inside the filter, and mechanical filtration depends on a reasonable ratio of performance and filter material (too large or dense a sponge can hinder operation).
The internal filter is mounted on the back or side wall of the aquarium in an inconspicuous place. Suction cups hold the device to the glass, and there are also special mounts allowing it to be fixed to the upper edge of the aquarium glass if your tank is not covered with a lid. It is permissible to lay the pump horizontally on the bottom and direct the jet upwards. This feature is especially useful in paludariums and small aquariums and terrariums where the water level is low. At the same time, it is necessary to prevent sand from entering the filter.
How long a small internal filter included in the kit will satisfy us depends on the quantity, or more precisely, the biomass of the fish and the frequency of water changes. An external filter works more reliably and stably in an aquarium. And it doesn’t have to be a large model; even small devices vastly outperform internal filters in volume.
Using internal filters, with air-lift, with pump, filter-pump.
Types of filtration. Chemical. Osmotic. Peat. Flotation. 
Last changes and additions were made on January 22, 2013.
